Short answer:
Dominant alleles/traits mask recessive alleles/traits. If both alleles are dominant, the organism will express that dominant trait. (i.e. RR)
If both alleles are recessive, the organism will express the recessive trait (i.e rr).
If one allele is dominant, and the other is recessive, it typically expresses the dominant trait (i.e.Rr).
Just in case you might need information, read on:
I'll take you back a bit just to help you understand the concept.
Genes carry the information that determines traits that you will get. Specific genes are found in specific locations of your chromosome (locus) and they come in two's and the pairs code for a specific trait. However, the pair of genes do not always match and this is what we call alleles.
Alleles are varied forms of genes. For example, you have a Black allele and a Brown allele for hair color, where Black is DOMINANT and brown is RECESSIVE.
We say dominant because it overrides the recessive if the gene has one of each in its pair. What happens then?
Well, the dominant masks the recessive. The recessive does not go away, but the dominant trait just has a greater effect, which cover the recessive trait . So if we have allele pairs that is black and brown, then the hair color would be black.
If the pair of alleles are both brown, then brown will be expressed because there is no black allele to cover it up.
